We have isolated a series of yeast secretory mutants (sec mutants) that are temperature-sensitive for cell surface growth, division, and secretion. Most of these mutants accumulate secretory proteins in an intracellular pool which can be released when cells are returned to a permissive temperature. At least 23 gene products have been implicated in the process of delivering membrane and secretory proteins to the cell surface. Electron microscope analysis has revealed three distinct membrane-bounded organelles that accumulate in different mutants. Ten of the mutants produce vesicles, nine of the mutants produce endoplasmic reticulum, and two of the mutants produce Golgi body-like structures which we have called Berkely bodies. We have also identified nine genes that contrrl the earliest steps in the cell surface growth pathway. Mutations in these genes result in a failure to synthesize active secretory protein, in spite of a normal rate of overall protein synthesis. Two of the mutants are also defective in the production of a membrane permease activity. The secretory mutants are now being used to identify some of the molecular events involved in cell-surface growth. In higher eukaryotic cells, glycoproteins are glycosylated in stages during their transit to the cell surface and thus if a secretory mutant blocks passage from one stage to another, incompletely glycosylated forms may accumulate. We have detected at least two stages in oligosaccharide assembly on the secretory enzyme invertase. The secretory mutants that accumulate endoplasmic reticulum at a nonpermissive temperature produce an incompletely glycosylated form of invertase.